It was <2020-05-19 wto 12:43>, when Russell King - ARM Linux admin wrote: > On Tue, May 19, 2020 at 01:21:09PM +0200, Geert Uytterhoeven wrote: >> On Tue, May 19, 2020 at 11:46 AM Russell King - ARM Linux admin >> <linux@xxxxxxxxxxxxxxx> wrote: >> > On Tue, May 19, 2020 at 11:44:17AM +0200, Geert Uytterhoeven wrote: >> > > On Tue, May 19, 2020 at 10:54 AM Lukasz Stelmach <l.stelmach@xxxxxxxxxxx> wrote: >> > > > It was <2020-04-29 śro 10:21>, when Geert Uytterhoeven wrote: >> > > > > Currently, the start address of physical memory is obtained by masking >> > > > > the program counter with a fixed mask of 0xf8000000. This mask value >> > > > > was chosen as a balance between the requirements of different platforms. >> > > > > However, this does require that the start address of physical memory is >> > > > > a multiple of 128 MiB, precluding booting Linux on platforms where this >> > > > > requirement is not fulfilled. >> > > > > >> > > > > Fix this limitation by obtaining the start address from the DTB instead, >> > > > > if available (either explicitly passed, or appended to the kernel). >> > > > > Fall back to the traditional method when needed. >> > > > > >> > > > > This allows to boot Linux on r7s9210/rza2mevb using the 64 MiB of SDRAM >> > > > > on the RZA2MEVB sub board, which is located at 0x0C000000 (CS3 space), >> > > > > i.e. not at a multiple of 128 MiB. >> > > > > >> > > > > Suggested-by: Nicolas Pitre <nico@xxxxxxxxxxx> >> > > > > Signed-off-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx> >> > > > > Reviewed-by: Nicolas Pitre <nico@xxxxxxxxxxx> >> > > > > Reviewed-by: Ard Biesheuvel <ardb@xxxxxxxxxx> >> > > > > Tested-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx> >> > > > > Tested-by: Dmitry Osipenko <digetx@xxxxxxxxx> >> > > > > --- >> > > > >> > > > [...] >> > > > >> > > > Apparently reading physical memory layout from DTB breaks crashdump >> > > > kernels. A crashdump kernel is loaded into a region of memory, that is >> > > > reserved in the original (i.e. to be crashed) kernel. The reserved >> > > > region is large enough for the crashdump kernel to run completely inside >> > > > it and don't modify anything outside it, just read and dump the remains >> > > > of the crashed kernel. Using the information from DTB makes the >> > > > decompressor place the kernel outside of the dedicated region. >> > > > >> > > > The log below shows that a zImage and DTB are loaded at 0x18eb8000 and >> > > > 0x193f6000 (physical). The kernel is expected to run at 0x18008000, but >> > > > it is decompressed to 0x00008000 (see r4 reported before jumping from >> > > > within __enter_kernel). If I were to suggest something, there need to be >> > > > one more bit of information passed in the DTB telling the decompressor >> > > > to use the old masking technique to determain kernel address. It would >> > > > be set in the DTB loaded along with the crashdump kernel. >> > > >> > > Shouldn't the DTB passed to the crashkernel describe which region of >> > > memory is to be used instead? >> > >> > Definitely not. The crashkernel needs to know where the RAM in the >> > machine is, so that it can create a coredump of the crashed kernel. >> >> So the DTB should describe both ;-) >> >> > > Describing "to use the old masking technique" sounds a bit hackish to me. >> > > I guess it cannot just restrict the /memory node to the reserved region, >> > > as the crashkernel needs to be able to dump the remains of the crashed >> > > kernel, which lie outside this region. >> > >> > Correct. >> > >> > > However, something under /chosen should work. >> > >> > Yet another sticky plaster... >> >> IMHO the old masking technique is the hacky solution covered by >> plasters. > > One line of code is not "covered by plasters". There are no plasters. > It's a solution that works for 99.99% of people, unlike your approach > that has had a stream of issues over the last four months, and has > required many reworks of the code to fix each one. That in itself > speaks volumes about the suitability of the approach. As I have been working with kexec code (patches soon) I would like to defend the DT approach a bit. It allows to avoid zImage relocation when a decompressed kernel is larger than ~128MiB. In such case zImage isn't small either and moving it around takes some time. -- Łukasz Stelmach Samsung R&D Institute Poland Samsung Electronics
Attachment:
signature.asc
Description: PGP signature
